Today marks the 15th anniversary of the iPhone 4S launch, which famously introduced Siri as a built-in feature. The iPhone 4S went on sale on October 14, 2011, retaining the iPhone 4's design but with significant internal upgrades. Its most defining feature was Siri, an assistant that understood natural language for various tasks. Siri initially launched in beta and was exclusive to the iPhone 4S. This integrated Siri was developed from a previously existing standalone app that Apple acquired. Beyond Siri, the iPhone 4S sported a dual-core A5 chip for improved performance. It also featured an upgraded eight-megapixel rear camera capable of 1080p video recording. The device included a revised antenna system and doubled download speeds to 14.4 Mbps. The iPhone 4S launched with iOS 5, bringing features like iMessage and Notification Center. Apple announced over four million units sold in its first three days, a record at the time.